As a system for reducing force required for a driver to operate a steering wheel for providing a steering angle to steered wheels (which are generally front wheels except for special-purpose vehicles), a power steering system is widely used. As this power steering system, an electric power steering system which uses an electric motor as an auxiliary power source has recently widely spread. As the structure of this electric power steering system, various structures are known. In any structure, a rotating shaft is rotated by operating a steering wheel, and auxiliary power of an electric motor is applied to the rotating shaft to provide a steering angle to steered wheels according to the rotation, through a speed reducer. As this speed reducer, a worm speed reducer is generally used. In an electric power steering system using a worm speed reducer, a worm to be rotated by an electric motor is engaged with a worm wheel to rotate together with a rotating shaft, so that the auxiliary power of the electric motor can be transmitted to the rotating shall. In this worm speed reducer, if the rotation direction of the rotating shaft changes, due to backlash which is provided in the engagement portion between the worm and the worm wheel, unpleasant noise called tooth hitting noise may be generated.
As a structure capable of suppressing generation of tooth hitting noise, it has been considered to elastically press a worm toward a worm wheel by an elastic member such as a spring. FIGS. 8 and 9 show an example of an electric power steering system disclosed in Patent Document 1. A front end portion of a steering shaft 2 which is rotated in a predetermined direction by a steering wheel 1 is supported inside a housing 3 to be rotatable, and a worm wheel 4 is fixed to that portion. Worm teeth 5 to be engaged with the worm wheel 4 are provided on a middle portion of a worm shaft 6, and both axis-direction end portions of a worm 8 to be rotated by an electric motor 7 are supported inside the housing 3 by a pair of rolling bearings 9a and 9b such as deep groove type ball bearings such that the worm can rotate. A pressing piece 10 is fit on a portion of the tip end portion of the worm shaft 6 protruding from the rolling bearing 9a, and an elastic member such as a coil spring 11 is provided between the pressing piece 10 and the housing 3. The coil spring 11 presses the worm teeth 5 provided on the worm shaft 6 toward the worm wheel 4 with the pressing piece 10 interposed therebetween. According to this configuration, backlash between the worm teeth 5 and the worm wheel 4 is suppressed, and generation of tooth hitting noise is suppressed.
The above-described conventional structure can suppress tooth hitting noise from being generated at the engagement portion between the worm teeth 5 and the worm wheel 4 but cannot suppress noise from being generated at the joint portion between the tip end portion of the output shaft 12 of the electric motor 7 and the base end portion of the worm shaft 6. This point will be described below. In the structure shown in FIG. 9, in order to join the tip end portion of the output shaft 12 of the electric motor 7 and the base end portion of the worm shaft 6 such that torque can be transmitted, a spline hole 13 is formed in the base end portion of the worm shaft 6 so as to be opened on the base end surface of the worm shaft 6.
Meanwhile, a spline shaft portion 14 is formed at the tip end portion of the output shaft 12. Then, the spline shaft portion 14 and the spline hole 13 are spline-engaged, whereby the output shaft 12 and the worm shaft 6 are joined such that torque can be transmitted.
If the spline shaft portion 14 and the spline hole 13 are spline-engaged without a gap in a circumferential direction (without backlash), noise is not generated at the joint portion between the tip end portion of the output shaft 12 and the base end portion of the worm shaft 6 (the spline engagement portion). However, in actual products, at this spline engagement portion, backlash is provided. Especially, in a structure which uses the structure shown in FIG. 9 to suppress backlash between the worm teeth 5 and the worm wheel 4, since it is required to swing the worm shaft 6 to displace the worm shaft, backlash at the spline engagement portion cannot completely be eliminated, and it is difficult to prevent noise generation.
Patent Document 2 discloses a structure in which an output shaft of an electric motor and a worm shaft are joined with a columnar metal power transmission member interposed therebetween such that the worm shaft can be smoothly swung to be displaced. Even in the structure disclosed in Patent Document 2, at each of spline engagement portions of spline shaft portions (male splines) provided at both end portions of the power transmission member and spline holes (female splines) formed in end portions of the worm shaft and the output shaft of the electric motor for displacement of the worm shaft by swinging, backlash exists. Therefore, there is a possibility that noise will be generated when changing the rotation direction of the rotating shaft. Also, there is a possibility that, since the power transmission member is provided, the axis-direction dimension of the joint portion of the output shaft of the electric motor and the worm shaft may increase, resulting in increases in the size and weight of the whole electric power steering system.